Various types of implantable medical devices may be formed wholly or partially of biological tissue that has been chemically "fixed" or preserved. The technique used for chemical fixation of biological tissues typically requires exposure of the biological tissue to one or more chemical agents that are capable of forming cross-linkages between connective tissue protein molecules present in the tissue.
Examples of fixed biological tissues that have been used to form implantable bioprostheses include cardiac valves, blood vessels, skin, dura mater, pericardium, ligaments and tendons. These biological tissues typically contain connective tissue proteins (i.e., collagen and elastin) which act as the supportive framework of the tissue. The pliability or rigidity of each biological tissue is largely determined by the relative amounts of collagen and elastin present within the tissue and/or by the physical structure and confirmation of its connective tissue frame work.
Each Collagen molecule is made up of three (3) polypeptide chains intertwined in a coiled helical confirmation. The Chemical fixatives (i.e., tanning agents) which are used to preserve biological tissues generally form chemical cross-linkages between the polypeptide chains within a given collagen molecule (i.e., intramolecular crosslinkages), or between adjacent collagen molecules (i.e., intermolecular crosslinkages).
Examples of chemical fixative agents which have been utilized to cross-link collagenous biological tissues include; aldehydes (e.g., formaldehyde, glutaraldehyde, dialdehyde starch, para formaldehyde, glyceroaldehyde, glyoxal acetaldehyde, acrolein), diisocyanates (e.g., hexamethylene diisocyanate), carbodiimides, photooxidabon, and certain polyepoxy compounds (e.g., Denacol-810, -512, or related compounds). Of the various chemical fixatives available, glutaraldehyde is the most widely used. Glutaraldehyde is used as the fixative for many commercially available bioprosthetic products, such as porcine bioprosthetic heart valves (i.e., the Carpentier-Edwards.RTM. stented porcine bioprosthesis; Baxter Healthcare Corporation; Edwards CVS Division, Irvine, Calif. 92714-5686), bovine pericardial heart valve prostheses (e.g., Carpentier-Edwards.RTM. Pericardial Bioprosthesis, Baxter Healthcare Corporation, Edwards CVS Division; Irvine, Calif. 92714-5686) and stentless porcine aortic prostheses (e.g., Edwards.RTM. PRIMA Stentless Aortic Bioprosthesis, Baxter Edwards AG, Spierstrasse 5, GH6048, Horn, Switzerland).
One problem which has been associated with the implantation of bioprosthetic materials is that the connective tissue proteins (i.e., collagen and elastin) within these materials can become calcified following implantation within the body. Such calcification can result in undesirable stiffening or degradation of the bioprosthesis. Two (2) types of calcification-intrinsic and extrinsic--are known to occur in fixed collagenous bioprostheses, although the exact mechanism(s) by which such calcification occurs is unknown. Intrinsic calcification is characterized by the precipitation of calcium and phosphate ions within the fixed bioprosthetic tissue, including the collagen matrix and remnant cells. Extrinsic calcification is characterized by the precipitation of calcium and phosphate ions within the thrombus, including adherent cells (e.g., platelets) to the bioprosthesis and the development of calcium phosphate-containing surface plaques on the bioprosthesis.
The factors that affect the rate at which fixed tissue bioprostheses undergo calcification have not been fully elucidated. However, factors that are thought to influence the rate of calcification include:
a) patient's age; PA1 b) existing metabolic disorders (i.e., PA1 c) hypercalcemia, diabetes, etc.); PA1 d) dietary factors; PA1 e) infection; PA1 f) parenteral calcium administration; PA1 g) dehydration; PA1 h) distortion/mechanical factors; PA1 i) inadequate coagulation therapy during initial period following surgical implantation; and PA1 j) host tissue responses. PA1 a) tissue damage; PA1 b) diet; PA1 c) surface properties of the tissue, including the nature of exposed collagen (e.g., type I, IV, etc.); PA1 d) metabolic changes; PA1 e) coagulation; PA1 f) hemodynamics PA1 g) inflammation; and, PA1 h) infection. PA1 1. HARVEST/PREPARATION OF TISSUE--harvesting and preparing a biological tissue. PA1 2. FIXATION OF TISSUE--contacting the biological tissue with a fixative agent such as an aldehyde (e.g., formaldehyde or glutaraldehyde) for a fixation time period (e.g., when 0.625% glutaraldehyde is used as the fixative the time period will be 0.5 hours to 14 days) to effect crosslinking of the connective tissue proteins within the tissue. PA1 3. DSC TREATMENT--before or after performance of the fixation step (Step B above) the biological tissue is placed in contact with (e.g., immersed in) a denaturant/surfactant/crosslinking agent (DSC) solution at a temperature between 4 and 50 degrees C for a period of time ranging from 90 minutes to 35 hours). This DSC treatment step is typically performed before transportation of the biological tissue into a clean room or aseptic environment, but may additionally or alternatively be performed at other times such as after assembly of the bioprosthesis as described in more detail herebelow. PA1 4. ASSEMBLY/FABRICATION OF PROSTHESIS--trimming or configuring the biological tissue (if necessary) and attaching any required non-biological components (e.g., stents, frames, suture rings, other hardware elements) thereto. This assembly/fabrication step is typically performed in a clean room or aseptic environment; PA1 5. TERMINAL STERILIZATION--subjecting the biological tissue to a sterilant (e.g., a liquid sterilant such as 0.2-2.0% by weight glutaraldehyde solution) for a sterilization time period. A 0.625% glutaraldehyde solution may be used in combination with heat (i.e. warming above room temperature but below a temperature which would cause damage to the tissue of the bioprosthesis), as the sterilant. Alternatively, a suitable sterilant solution may comprise an osmotically balanced aqueous solution alone or in combination with a non-contacting source of sterilization (e.g., radiation, electron beam, UV, or other similar expedient), or include an aqueous solution of glutaraldehyde in combination with the above-described DSC mixture or some components of such DSC mixture. In instances where a 0.625% glutaraldehyde solution is used as the sterilant, the sterilization time period may be 1-6 days at 37 degrees C or 1-2 days at 50 degrees C). This terminal sterilization step may be performed after packaging of the bioprosthesis in its final container, thereby eliminating the need for any subsequent handling of the bioprosthesis until the time of implantation.
The factors that are thought to affect the propensity for platelets to adhere to a fixed bioprosthetic tissue include:
Various techniques have heretofore been proposed for mitigating the in situ calcification of glutaraldehyde-fixed bioprostheses. Included among these calcification mitigating techniques are the methods described in U.S. Pat. No. 4,885,005 (Nashef et al.) entitled Surfactant Treatment of Implantable Biological Tissue To Inhibit Calcification; U.S. Pat. No. 4,648,881 (Carpentier et al.) entitled Implantable Biological Tissue and Process For Preparation Thereof; U.S. Pat. No. 4,976,733 (Girardot) entitled Prevention of Prosthesis Calcification; U.S. Pat. No. 4,120,649 (Schechter) entitled Transplants; U.S. Pat. No. 5,002,2566 (Carpentier) entitled Calcification Mitigation of Bioprosthetic Implants; EP 103947A2 (Pollock et al.) entitled Method For Inhibiting Mineralization of Natural Tissue During Implantation and WO84/01879 (Nashef et al.) entitled Surfactant Treatment of Implantable Biological Tissue to Inhibit Calcification; and, in Yi, D., Liu, W., Yang, J., Wang, B., Dong, G., and Tan, H.; Study of Calcification Mechanism and Anti-calcification On Cardiac Bioprostheses Pgs. 17-22, Proceedings of Chinese Tissue Valve Conference, Beijing, China, June 1995.
There presently remains a need in the art for the development of new calcification-mitigating methods for fixing (i.e., tanning and crosslinking) and sterilizing biological tissues to provide bioprosthetic devices which are a) less likely to become calcified and b) less thrombogenic, following implantation within a patient's body.